Project Summary: Muscles contribute directly to in vivo loading of the femur and normally protect the bone from exposure to damaging levels of shear and tensile stresses and strains. Age-related changes in the muscle loading experienced during daily activities could increase the rate of fatigue damage in the proximal femur, thus increasing the risk of hip fracture. The proposed research will investigate the effect of muscular loading on stresses and strains in the proximal femur during normal walking using a unique computational model. The first aim of this project is to investigate age-related changes in femoral stresses and strains due to muscular loading during gait. The second aim is to quantify the contributions of individual muscles to femoral stresses and strains during gait. This information will contribute to understanding of the causes of hip fractures, and in the long term could be used to improve diagnostic and preventive measures, decreasing the incidence of these debilitating injuries. A computational model will be developed to estimate the stresses and strains in the proximal femur during gait. This model will combine two computational techniques that are widely used in musculoskeletal biomechanics: static optimization and finite element modeling. Kinematics and ground reaction force data during gait will be recorded for 10 subjects ages 18-25 and 10 subjects ages 65-80. This data will be analyzed using inverse dynamics and static optimization to determine realistic values for the individual muscle forces acting on the femur throughout the gait cycle. Finite element models of the femur will be developed and, with the estimated muscle forces, used to predict the stresses and strains in the proximal femur during gait. The peak stresses/strains will be compared between age groups, and a sensitivity analysis performed to determine the importance of each muscle to the overall stress/strain state. Relevance: Hip fractures are one of the most serious healthcare problems facing older adults today due to high rates of post-fracture death and disability. This research will examine how age related changes in muscle loading during gait may contribute to the weakening and eventual fracture of the bone. Such information could lead to improved screening techniques and preventive measures to reduce the incidence of hip fractures.